The Site of Spermatozoon Entrance in the Unfertilized Egg of Discoglossus pictus (Anura): An Electron Microscope Study

Abstract

The egg of Discoglossus pictus has an animal dimple, a pit located at the animal hemisphere pole, which is the only site successfully penetrated by the sperm. Ultrastructural analysis of the unfertilized egg surface reveals unique features characterizing the animal dimple. They are: I) regularly spaced, finger-like microvilli supported by bundles of fibers which appear to be microtubules and surrounded by antennular glycocalyx, present at the surface of the animal dimple. The orientation and distribution of the fibers suggest that they may play an essential role in maintenance of the shape of the animal dimple: 2) granules present in the peripheral cytoplasm of the animal dimple whose characteristics are somewhat different from those of the typical Anuran cortical granules and which are absent from the rest of the peripheral cytoplasm of the egg. The unique presence of these granules at the animal dimple and their apparent loss at the time of fertilization suggest that they might actively participate in the fertilization process. Freshly laid eggs of Discoglossus picius (painted frog) exhibit a depression (the animal depression) at the animal hemisphere whose center part is invaginated and forms the animal dimple. The animal depression is filled with a gelatinous lens-shaped material forming the animal plug, which protrudes at the surface of the egg (Hibbard, 1928). The eggs are further covered with four gelatinous envelopes with different chemical compositions. The innermost envelope is made up of two membranous layers E1 and E2 and strictly adheres to the egg surface, except over the animal dimple where they are separated from the egg surface by a substance of glycoprotein nature (Ghiara, 1960). The nucleus, arrested in metaphase of the 2nd meiotic division, is located in the cortical cytoplasm of the area germinativa which is the bottom of the animal dimple. In sections of eggs at the level of the animal dimple, and particularly at its basal portion, one can clearly distinguish striations which are perpendicular to the egg surface (Hibbard,